Process for the production of DMT-intermediate product of specific purity

ABSTRACT

The invention relates to a process for the production of a DMT intermediate product as well as its working up to fibregrade DMT and to medium pure and pure terephthalic acid. The DMT intermediate product is moreover produced by combined oxidation of a predominantly para-xylene (p-X) and para-toluic acid methyl ester (p-TE) containing mixture in liquid phase with an oxygen containing gas, esterification of the acids being produced with methanol, distillative or rectificative separation of the esterification products being produced into a p-TE rich fraction I, a fraction II containing more than 99% by weight DMT and its isomers and a high boiling residue fraction III as well as feeding back the p-TE fraction I into the oxidation, with the residue fraction III having a DMT content of 15 to 70% by weight and the DMT fraction II being purified by single solvent recrystallisation in methanol to form the DMT intermediate product to such an extent that the amounts of hydroxymethylbenzoic acid methyl ester (HM-BME) and terephthalic acid methyl ester (TAE) in the DMT intermediate product amount together to less than 200 ppm.

FIELD OF THE INVENTION

The invention relates to a process for the production ofDMT-intermediate product according to the preamble of claim 1 as well asits working up to medium pure or fibregrade dimethylterephthalate(DMT-p) and/or medium pure or pure terephthalic acid (MTA/PTA).

Dimethylterephthalate (DMT) is produced in numerous large scale plantsaccording to the so-called Witten-DMT-Process (cf. German PatentSpecification 10 41 945). DMT is subsequently worked up to polyesters byreaction with multifunctional alcohols. Those high molecularcompounds--also known as saturated polyesters--are worked up inter aliato form fibres, filaments, films or moulded parts.

STATE OF THE ART

According to the Witten-DMT-Process, a mixture of para-xylene (p-X) andpara-toluic acid methylester (p-TE) is oxidised in the liquid phase inthe absence of solvents and of halogen compounds at a pressure of about4 to 8 bar and a temperature of about 140° to 180° C. with atmosphericoxygen in the presence of dissolved heavy metal oxidation catalysts,e.g. in the presence of a mixture of cobalt and manganese compounds (cf.German Patent Specification 20 10 137).

Subsequently to the oxidation step, the reaction mixture obtained, whichconsists predominantly of monomethyl terephthalate (MMT), p-toluic acid(p-TA) and terephthalic acid (TA), dissolved or suspended, resp., inpara-toluic acid methyl ester (pT-ester) and dimethyl terephthalate(DMT), is esterified with methanol at a pressure of 20 to 25 bar and atemperature of about 250° to 280° C. Higher pressures are technicallypossible but for cost reasons are not used. The esterification productis distillatively separated into a p-TE-fraction, a crude DMT fractionand a high boiling, tar-like residue. The p-TE-fraction is fed back intothe oxidation. The high boiling, tar-like distillation residue contains,inter alia, all the constituents of the catalyst system, which accordingto EP-B-0 053 241 can be recovered and fed back into the oxidation.

The crude DMT originating from the distillation and having a typicalpurity of 97 to 99.9% contains, besides about 0.05 to 2% of isomers ofDMT (dimethylortho- and dimethylisophthalate [DMO, DMI]), still partlyinterfering amounts of terephthalaldehydic acid-methylester (TAE),monomethylterephthalate (MMT), para-toluic acid (p-TA) and otherimpurities resulting from the p-xylene employed or from side reactions.

For purification of the crude DMT to pure fibregrade) DMT (DMT-p), withit being necessary in particular, for TAE and the isomers to be removed,it is known to treat the crude DMT by methanolic recrystallisation(German Offenlegungsschrift 20 14 012, Hydrocarbon Processing, Nov.1983, P. 91). For attaining a purity of the DMT (sum of impuritiesinclusive of the isomers DMO and DMI less than 100 ppm) sufficient forfibre production (fibregrade DMT), it has been to some extent usual tocarry out the recrystallisation twice with intermediate washing withmethanol, with the methanol being fed in countercurrent. In addition afinal distillation of the DMT has still been necessary hitherto with acrude DMT of low purity.

The filtrate residue obtained in the methanolic recrystallisation byfiltration from the mother liquor still contains, in addition to theisomers DMO and DMI, large amounts of DMT and other valuable products(intermediate products in the DMT-process), so that the filtrationresidue was hitherto as a general rule for the largest part fed backinto the oxidation. As a result of this feeding back of filtrationresidue, the isomers accumulate up to values of 8 to 12% by weight sothat the antecedent reactors and especially the distillation andrecrystallisation must have additional capacities. The feeding back ofthe filtration residue was, according to the state of the art, necessaryfor obtaining sufficiently "neutral prcducts" for the adjustment of themelting point and the acid number in the oxidation and the flowabilityof the oxidation product to the esterification.

From German Patent Specification 30 11 858 and German PatentSpecification 29 16 197 it is known that the amount ofterephthalaldehydic acid methylester (TAE) in the DMT intermediateproduct must be kept as small as possible, in particular in the workingup to form fibregrade terephthalic acid (PTA). In German PatentSpecification 30 11 858, there is proposed a particularly expensiverectification column, for reduction of the TAE content in the crude DMT,in which column the TAE content in the crude DMT can be reduced tovalues of <0.01% by weight.

In spite of this high cost, it has been shown with this process that analways still interfering large amount of terephthalaldehydic acid (TAS,4-CBA), the acid formed from the TAE in the hydrolysis, can be presentin the end product, terephthalic acid (TA). The residual content of TASin the terephthalic acid could then assume in particular inexplicablyhigh values if the crude DMT had to be stored in the meanwhile beforethe working up to terephthalic acid.

OBJECT OF THE INVENTION

It is the object of the present invention to improve the process of theindicated type for the production of DMT intermediate product to theeffect that, in the working up of the DMT intermediate product tofibregrade DMT, the TAE content, and in the working up to PTA, the TAScontent, is reduced further. In particular, even with longerintermediate storage of the crude DMT and then working up to pure DMT orPTA respectively, the TAE content or the TAS content, in the end productshould be kept as low as possible.

Further concerns of the invention are the lowering of the cost ofapparatus and energy in the DMT and PTA production, the increasing ofyield and the creation of a possibility of producing DMT-p and PTAsimultaneously in one plant.

SUMMARY OF THE INVENTION

This object is solved according to the invention by the features ofclaim 1.

It is an essential feature of the invention to improve and to simplifythe purification of the crude DMT by cost effective rectification andsubsequent single solvent recrystallisation.

The largest part of the impurities--with exception of the isomers--isremoved according to the invention by rectification so that the thuspurified crude DMT (fraction II) preferably has a TAE andhydroxymethyl-benzoic acid methylester content (HM-BME) of together lessthan 0.2% by weight (2.000 ppm). The rectification of the ester mixturecan be simplified, in relation to the process according to German PatentSpecification 30 11 858 because the high boiling residue fraction IIIhas a high DMT content of 15 to 70% by weight, preferably 40 to 60% byweight.

Likewise, the p-TE rich fraction I can have a high DMT content, thislying preferably at 20 to 30% by weight.

The residue fraction III can be prepared for example with a processaccording to German Patent Specification 24 27 875. According to apreferred embodiment of the invention, the residue fraction is howeverprepared by methanolysis in two steps, with the DMT as well as furthervaluable products being fed back into the oxidation. In all, the DMTcontent, which is recovered from the residue fraction and is fed backinto the oxidation, amounts preferably to 3 to 7% by weight related tothe sum of all the components supplied in the liquid phase to theoxidation.

Preferably, the p-TE fraction I is fed back in excess to the oxidationso that still about 5 to 30% by weight of p-TE, in particular 20 to 25%by weight, are obtained in the acid mixture being produced in theoxidiser.

The adjusted p-TE excess as well as the DMT quantities recycled servefor increasing reaction and for melting point adjustment of the productflow leaving the oxidiser. Melting points of 130° to 150° C. with anacid number of 200 to 300 g KOH/kg oxidate are preferred.

The crude DMT purified by rectification (fraction II) is then subjectedto a single solvent recrystallisation, preferably in methanol, in orderto remove residual amounts of TAE, HM-BME and other impurities. At thesame time, the isomers DMO and DMI can be removed in this step.

According to a preferred embodiment of the invention, the mother liquororiginating from the (first) methanolic recrystallisation isconcentrated by evaporation of methanol. The dissolved DMT with amountsof isomers and impurities corresponding to the solvent equilibrium arethen crystallised out as a result of further cooling to about 20° to 50°C., preferably about 25° C., filtered off and after mixing with methanolfed back to the (first) recrystallisation. The substances stilldissolved in the remaining mother liquor, predominantly DMO, DMI andresidues of DMT as well as the amounts of TAE and HM-BME not removed inthe rectification, are concentrated by evaporation of the residualmethanol and discharged as residue from the process or fed back into theoxidation.

Insofar as is usual according to the state of the art, the p-xyleneemployed has an isomer content of above 1,000 to 5,000 ppm, according toa preferred embodiment of the invention, a feeding back into theoxidation of the residue being deposited is completely dispensed with. Acomplete discharging of the residue is therefore not subject touneconomical losses because, in the preceding rectification, thevaluable products TAE and HM-BME have already been removed almostcompletely from the crude DMT. At the same time, a feeding back ofresidue--in contrast to the known process--is also not necessary for theadjustment of the melting point and the acid number in the oxidisersince the neutral product quantities needed for this purpose are fedback into the oxidation as DMT-proportions of the p-TE-fraction I andthe residue fraction III.

With p-xylene particularly clean with respect to isomers, with isomeramounts below 100 to 500 ppm, the entire residue can optionally also befed back into the oxidation, insofar as a corresponding amount ofisomers can be tolerated in the end product. With these small amounts ofisomers in the p-xylene, which according to the present state of the artis certainly not in sufficient amounts for convenience, the amount ofresidue fed back is however relatively small, amounting to about 3 to 6%by weight of the amount of DMT fed back into the oxidation from therectification.

Indeed, depending on required end product purity and amount of isomersin the p-xylene, a part of the filtrate residue can also optionally befed back into the oxidation, with the amount of DMT and its isomersbeing small in relation to the amounts of DMT, which are fed back intothe oxidation from the rectification.

It is a further essential feature of the invention to minimise also theamount of the impurity hydroxymethyl-benzoic acid methylester (HM-BME)present in the DMT intermediate product, in addition to TAE.

It has been surprisingly shown that the amount of HM-BME in the crudeDMT, in addition to the amount of TAE, is decisive for the TAE contentin the fibregrade DMT or for the amount of TAS in PTA. It is supposedthat HM-BME in the intermediate storage tank connected to the DMTproduction or in the subsequent processing steps is reacted withatmospheric oxygen to form TAE, with the atmospheric oxygen being ableto be present in the closed plant as a result of unavoidable microleakages in the distillation part, which is run with sub-atmosphericpressure. This relationship was unknown until the time of the invention.

As a result of the process according to the invention for the productionof DMT intermediate product, it has been possible for the first time toessentially simplify the subsequent processing steps for producingfibregrade DMT and/or PTA, without the TAE content in the pure DMT orthe TAS content in PTA assuming higher values. Also, with longer interimstorage of the DMT intermediate products, it can be guaranteed reliablythat the amount of TAE or TAS in the end product does not increasefurther, so that a uniform quality is attained.

The DMT intermediate product according to the invention with a typicalcontent of isomers, TAE and HM-BME of, in total, 50 to 200 ppm can beworked up by a second recrystallisation in methanol with low cost inapparatus to fibregrade DMT with a typical purity of >99.995% by weight.The content of impurities may be reduced, when needed, once again tovalues of <10 ppm by means of an additional methanolic washing of theDMT intermediate product before the final recrystallisation.

The working up of the DMT intermediate product to terephthalic acidtakes place preferably by hydrolysis in a simple reactor, the producedmethanol being removed together with the by-product dimethylether (DME)by feeding in of stripping steam. The terephthalic acid formed isdischarged from the reactor suspended or dissolved in water andcrystallised out in multiple stages, centrifuged and dried. Insofar asthe DMT intermediate product is supplied to the hydrolysis directlywithout additional water, the medium pure terephthalic acid (MTA)typically contains 500 to 1,000 ppm of impurities. By means of a simplewashing of the DMT intermediate product in methanol and subsequentcentrifuging, the purity of the terephthalic acid may be increased to100 to 150 ppm (PTA), with the impurities chiefly consisting of MMT. Asa result of simple washing of the PTA in water, even the MMT content,insofar as necessary, may be further reduced so that a terephthalic acidwith a purity hitherto hardly attainable in a large scale industrialplant with a sum of all impurities inclusive of MMT and p-TA of <50 ppmcan be produced (PTA-p).

Furthermore, an essential saving of energy is achieved with the processaccording to the invention. Of especial value is the possibility ofworking up the DMT intermediate product according to the invention bothto fibregrade DMT (DMT-p) and also to medium and pure terephthalic acid(MTA or PTA) with this working up being essentially simplified, forexample in comparison to the DMT intermediate product according toGerman Patent Specification 29 16 197 and German Patent Specification 3011 858, since the DMT intermediate product according to the inventionhas only very small residual contents of isomers. Finally, the processaccording to the invention opens up the possibility of producingterephthalic acid of very high purity (PTA-p) with, in contrast toconventional PTA, reduced amounts of impurities, in particular even ofMMT and p-TA.

Of special advantage is finally the possibility of producing withreduced additional cost of apparatus and energy at the same time pureDMT and fibregrade or very high purity terephthalic acid, with theproportions of DMT-p/PTA and PTA-p being able to be varied as desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the invention is explained in the following bymeans of explanatory examples as well as the drawings.

There are shown herein as plans for plant:

FIG. 1 a plant for the production of crude DMT with oxidiser,esterification column and crude ester rectification,

FIG. 2 a plant for residue working up of the sump fraction "Residue A"according to FIG. 1,

FIG. 3 a plant for the purifying of the crude DMT fraction from FIG. 1to form DMT intermediate product and fibregrade DMT,

FIG. 4 a plant for the filtrate working up of "Filtrate A" according toFIG. 3,

FIG. 5 a plant for the hydrolysis of DMT intermediate product to formPTA,

FIG. 6 a plant for the simultaneous production of PTA and PTA-p.

WAYS OF CARRYING OUT THE INVENTION EXAMPLE 1

In FIG. 1 there is shown a plant for the production of crude DMT(fraction II) according to the process of the invention with the maincomponents oxidiser 1, esterification column 5, and crude esterrectification 7 and 11. An analysis of the material flows 1.1 to 1.20defined subsequently and shown in FIG. 1 is indicated in Table I.

In the oxidiser 1, known per se, for example corresponding to DE-C3-2805 915, a liquid mixture consisting chiefly of p-xylene (material flow1.1) and p-TE (material flow 1.4) is oxidised with atmospheric oxygen(material flow 1.2) at a temperature of about 160° C. and a pressure ofabout 7 bar and subject to addition of catalyst (material flow 1.3).

The oxidation product P1 being produced contains as main componentsp-toluic acid (p-TA) (about 17% by weight), MMT (about 22% by weight),p-toluic acid methylester (p-TE) (about 24% by weight), DMT (about 14%by weight) and terephthalic acid (TA) (about 10% by weight).

Moreover, by-products and impurities, in particular benzoic acidmethylester (BME), TAE, HM-BME, high boiling substances etc., are to befound.

In the oxidation, the oxygen contained in the air (material flow 1.2) isconsumed up to a residual content. The otherwise remaining nitrogen issaturated with the substances to be found in the oxidiser 1 and leavesas oxidiser discharge gas (material flow 1.5) the oxidiser 1 togetherwith the reaction water being formed in the oxidation, as well as highboiling cracking products like CO, CO₂, formic acid and acetic acid.

The xylenic mixture denoted as p-xylene (material flow 1.1) consists offreshly employed xylene with an isomer content of 6,000 ppm as well asthe xylenic and organic fractions respectively recovered from thematerial flows 1.5, 1.6 and 1.10.

The para-toluic acid methylester (p-TE) (material flow 1.4) is withdrawnfrom an intermediate tank which is not shown which is fed predominantlyfrom the head product of the crude ester rectification column 11(material flow 1.12). In the exemplifying embodiment according to FIG.1, the organic components of the flows 1.11, 1.16, 1.15 as well as theproducts recovered from the residue 1.14 are likewise fed into the p-TEintermediate tank.

In the main, the p-TE-flow 1.12 is accordingly fed back into theoxidiser 1 so that a closed circuit exists.

The reaction products being produced in the oxidation have in part highmelting points--p-TA of about 180° C., MMT of about 227° C.--or arepractically not meltable (terephthalic acid) and are soluble in othersubstances only to a limited extent so that the danger of acrystallising out and accordingly blockage occurs. As a result of arelatively high amount of p-TE and DMT running in the circuit, thisdanger can be restricted.

The p-TE-excess employed as well as the DMT quantities running in thecircuit serve to increase reaction and adjust melting point of theproduct flow P₁ leaving the oxidiser. Melting points in the region of140° C. are preferred in this connection.

The acid number, which is held at 200 to 300 g KOH/kg oxidation productat the outlet from the oxidiser, is a valid measure of the well-balancedratio between high melting acids and other products improving themelting point and flowability. With too high acid numbers, badflowability and disadvantageous melting points are to be expected andwith too low acid numbers, that is too large amounts of circulatingproducts, too high distillation cost in the crude ester distillationcolumns 7 and 11. The p-TE-excess is so adjusted that the oxidationconditions in the oxidiser remain moderate, that is the temperature liesbetween 150° and 180° C. at 5 to 8 bar. It has been established thatconcentrations of 5 to 30% by weight p-TE should be established in thefinal oxidation product. The quantitative flows shown in Table I areobtained if a small p-TE excess of 24% and an acid number at the outletfrom the oxidiser of 225 g KOH/kg oxidation product are maintained.

The oxidation product produced is freed of xylene still containedtherein in the stripper 2 by means of steam (material flow 1.7). Thevapours which are obtained have a composition 1.6 according to Table I.

The oxidation product flow P₁ is next after increasing the pressure bymeans of the high pressure pump 3, combined with hot methanol (materialflow 1.8) and, after heating in the heat exchanger 4 to 250° C., isesterified in the esterification reactor 5 at 250° C. with methanol(material flow 1.9). The methanol is supplied at the sump in the form ofvapour. The relatively high methanol excess leaves the head of thecolumn 5 (material flow 1.10) with the reaction water and is saturatedwith other compounds present at the head of the column 5. The esterifiedoxidation product, the so-called crude ester, is depressurized to normalpressure in the intermediate tank 6 and is moreover cooled totemperatures of about 200° C. The methanol still present escapes in theform of vapour (material flow 1.11) via a vapour duct and is saturatedwith the components obtained in the reactor.

Crude Ester Rectification

The crude ester mixture, which still contains by-products like TAE, BME,high boiling substances, HM-BME as well as the catalyst, in addition toDMT (about 58% by weight) and p-TE (about 33% by weight), is pumped intothe rectification consisting of two columns 7, 11 from the intermediatecontainer 6.

The high boiler column 7 with vaporiser 8 and condenser 9 serves fordriving off of high boiling substances contained in the crude ester(material flow 1.14) as sump product, which can be subsequently treatedas "Residue A" in a separate residue working up (FIG. 2).

In addition, the "Residue A" contains all of the catalyst as well as ahigh proportion of about 52% by weight DMT. The largest part of theHM-BME is likewise discharged together with the residue fraction.

As a result of the provided high DMT content in the sump flow 1.14 ofthe high boiler column 7, the sump temperature can be held at 250° C.and the head pressure at 0.2 bar, with a high temperature level of about205° C. which makes possible an energetically satisfactory recovery ofthe condensation heat (condenser 9) in the form of low pressure steam(3.5 bar) being set at the head of the high boiler column 7. The headpressure of about 0.2 bar in the high boiler column 7 is maintained by avacuum plant which is nor shown, wherein the material flow 1.16 isdischarged.

The condensed head product flows from the condenser 9 in the low boilercolumn 11 with vaporiser 12 and condenser 13 in order to drive off thep-TE fraction. At head temperatures of about 194° C., the condensationheat can be used to produce 3.5 bar low pressure steam. The DMT contentin the head product is held high at about 18% by weight so that,inclusive of the amounts of DMT contained in the residue A (materialflow 1.14), there is obtained a high circulating amount of DMT andaccordingly the acid number of 225 g KOH/kg oxidation product in theoxidate. The relationships set in the exemplifying embodiment shown areobtained from Table I (material flow 1.14 and 1.12). Shouldrelationships which are suitable for the process be set by the choice ofthe refluxes and amounts, the amounts of TAE and HM-BME in the crude DMTof 0.1% by weight for TAE and 0.05% by weight for HM-BME respectivelyindicated in Table I are attained.

A sump temperature of 250° C., a head temperature of 194° C. and a headpressure of 0.3 bar are set in the low boiler column 11. The headpressure is maintained by pumping off the flow 1.15 by means of a vacuumplant which is not shown.

The high p-TE quantity obtained at top of column 11 assures a residualcontent of p-TE in the oxidation product P1 of 24%. The substancesHM-BME and TAE important for the quality of crude DMT representintermediate products of the p-TE oxidation to MMT and can therefore befed back to the oxidiser with the p-TE distillate and the valuableproducts obtained from the residue respectively without problem. It isapparent from the reflux ratios or the amounts of vapour in the columns(7,11) that the energy necessary for the distillation is relatively lowcompared with the process according to DE-C1-30 11 558. In particular itis advantageous that the condensation heat of the low boiler column 11can be utilised in the form of low pressure steam (3.5 bar).

Recrystallisation

The crude DMT (fraction II) purified according to FIG. 1 byrectification is further purified to the DMT intermediate product bysingle methanolic solvent recrystallisation (FIG. 3, analyses of thematerial flows 3.1 to 3.5, see Table III). Moreover the crude DMT(material flow 3.1, correspondingly material flow 1.13 according toFIG. 1) is mixed up in the mixing tank 28 with methanol from theintermediate tank 39. The hot solution is cooled in the crystalliser 29by means of rotary pump 30 and condenser 31 by release of pressure. Thepump 32 conveys the crystalysate to the centrifuge 33 where it isseparated into the Filtrate A and the crystalysate. The Filtrate A isfed via tank 34 and pump 35 (material flow 3.4) to the isomer dischargesystem (FIG. 4)

The separation as completely as possible of the crystallisation productand mother liquor adhering thereto is of decisive significance for thepurification effect of the apparently expensive recrystallisation since,with respect to the mother liquor, this is subject to almost all theimpurities. A methanolic washing of the crystalysate is thereforecarried out in the illustrated exemplifying embodiment to increase thepurification effect of the recrystallisation. Moreover, the crystalysateis mixed in the tank 36 with freshly distilled methanol and supplied viaa pump 37 to the centrifuge 38. The filtrate being produced--an onlyslightly impure methanol--is supplied to the head of the melter 41,while the crystalysate is conducted into the melter 41.

In the melter 41, the residual mother liquor is driven off from thecrystalysate and the DMT melted. The vaporised methanol is condensed inthe head of the melter 41 and fed together with the filtrate from thecentrifuge 38 into the intermediate tank 39 for methanol.

The DMT (material flow 3.5) taken from the melter 41 via the pump 42represents the DMT intermediate product of desired purity. It can as amatter of choice be worked up by hydrolysis (FIGS. 5 and 6) toterephthalic acid or can be worked up by a second recrystallisation tofibregrade DMT (DMT-p), with the combination of these possibilities, inparticular offering surprising advantages. The DMT intermediate product(material flow 3.5) still contains only very little impurities of TAEand HM-BME, of a maximum of 100 ppm, in the indicated explanatoryexample of only 17 ppm (see Table III), as well as small amounts of theisomer DMI and DMO, in the illustrated exemplifying embodiment, together67 ppm.

Processing of filtrate

The filtrate removed from the intermediate tank 34 (FIG. 3) via the pump35 (material flow 3.4, FIG. 3=material flow 4.1, FIG. 4) is initiallyconcentrated by evaporation in the vaporiser 59 with separator 60 sothat the concentrated mixture consists of up to 2/3 of methanol and 1/3of dissolved substance (DMT+impurities). The evaporation pressure is 4bar and the temperature is 100° C. The remaining solution is storedunder this pressure in the tank 61 and pumped by means of a pump 62 intothe crystalliser 63 with condenser 64, where the solution is cooled byevaporation cooling to about 25° C.

The corresponding suspension is supplied via pump 65 to the centrifuge66 and separated into a filtrate and the crystalysate. The crystalysatecan, as shown in FIG. 4, be fed back via pump 68 to the firstrecrystallisation as suspension mixed in stirrer tank 67 with methanol(material flow 4.3 in FIG. 4, material flow 3.3 in FIG. 3).

The filtrate is conducted into the collecting tank 69 and from there bymeans of the pump 70 to the vaporiser 71 in which the methanol isevaporated off completely (material flow 4.5, Table 4). The remainingmixture consists predominantly of DMI (37% by weight) and DMO (26% byweight) and contains only relatively small amounts of valuable products,in particular DMT (16% by weight), TAE (10% by weight) as well as 5% byweight each of p-TA and HM-BME so that they are discharged completelywithout great losses via the pump 72 (material flow 4.4), in particularto undergo combustion.

Processing of residue

"Residue A" according to FIG. 1 (material flow 1.14, Table I=materialflow 2.1, Table II) is stored in tank 15 (FIG. 2). Residue A contains,in addition to DMT (about 52% by weight), predominantly high boilingsubstances (37% by weight) as well as the catalyst (about 1% by weight).

The acids still present in Residue A are afterwards esterified in thetwo-step methanolysis according to FIG. 2, a part of the high boiling,in particular binuclear aromatic, compounds are cleaved and the valuableproducts (DMT, HM-BME, methoxymethylbenzoic acid methylester [MM-BME]and p-TE) are separated from the remaining undesired high boilingsubstances. The valuable products are fed back into the oxidation whilethe undesired high boiling substances are discharged from the process.

Among the distillable high boiling substances found in residue A aresubstances which, insofar as they are fed back into the oxidation,influence yields or selectivity of the oxidation disadvantageously sincethey influence the effectiveness of the catalyst system. Moreover suchsubstances can, during the oxidative treatment, form cleavage productse.g. CH₃ -DMT isomers, which are disadvantageous for the quality of thecrude DMT, can only be removed distillatively with difficulty from thecrude ester and require additional purification expenditure in thesubsequent crystallisation if high purity polyester crude substances areto be produced. The feeding back of such high boiling substances intothe oxidation must therefore be avoided.

The following reaction guidance is given in the illustrated exemplifyingembodiment:

Residue A from the crude ester distillation is run from the tank 15 bymeans of the pump 16 into a circulating system which consists of pump18, heat exchanger 17 and reactor 19. At the same time, methanol in theform of vapour (material flow 2.10) is pumped into the circulation. As aresult of the presence of the methanol, residual amounts of esterifiedhigh boiling acids which are still contained are esterified and to someextent taken up by the high methanol vapour excess and flashed in theform of vapour into the vapour space of the reactor 19. The amount ofliquid which is not vaporised remains in the sump part of the reactor 19and is treated afresh with methanol vapour and cleaved methanolyticallyagain. The esters being produced remain in the methanolic vapour phaseand are conducted together with the distillable constituents of theresidue and the flash product into the distillation column 20. Thetemperature in the reactor 19 chosen in the exemplifying embodimentamounts to 265° to 270° C., whereas the temperature in column 20 is atabout 250° C., so that a part of the high boiling constituents can bewithdrawn at the sump of the column 20. The high boiling by-products areconcentrated in the operating part of the distillation column 20, andwithdrawn at the sump of the column. A part of the high boiling productis discharged, while the rest, mixed with methanol vapour, is fed backvia pump 22 and vaporiser 21 to the sump of the column. Until thedistillable high boiling substances, which do not represent valuableproducts and therefore, as mentioned above, leave the sump of the column20 as high boiling substances, all valuable products are directed withthe methanol vapour to the dephlegmator 23 and condensed. A part of thecondensate is used as reflux, whereas the rest is discharged (materialflow 2.5) and is mixed with the p-TE flow for the oxidation. Themethanol excess (material flow 2.4) is directed together with the waterbeing produced in the methanolysis and post-esterification to a methanoldewatering column (not shown).

A part of the circulating flow from the reactor 19 is discharged fromthe circuit and subjected to a further methanolysis step in the reactor24 with heat exchanger 25, circulating pump 26 and dephlegmator 27. Thissecond methanolysis step is carried out at a temperature of >275° C., sothat further high molecular compounds of Residue A are split off. Theproducts leaving the reactor 24 in gaseous form are condensed completelyin the dephlegmator 27, with the exception of the methanol, while themethanol (material flow 2.7) together with the methanol from thedephlegmator 23 is discharged for reuse in the general process. Theproducts condensed in the dephlegmator 27 are for the largest part fedback together with the p-TE flow into the oxidation, while a small partis fed back as reflux to the head of the column of the reactor 24. Theconstituents which have not vaporised are withdrawn at the sump of thereactor 24 and discharged partly for catalyst recovery (material flow2.3) while the remainder is led back by means of pump 26 via vaporiser25 to the reactor. In addition methanol in the form of vapour issupplied to the reactor circulation.

EXAMPLE 2

The washing of the DMT crystalysate can possibly be dispensed if notespecially high requirements are placed on the purity of the DMTintermediate product, e.g. for the production of medium pureterephthalic acid (MTA). In FIG. 3, this alternative for the productionof DMT intermediate product is shown in broken line. The DMTintermediate product (product flow 3.7) withdrawn from the melter stillcontained about 150 ppm of TAE and HM-BME as well as 580 ppm of isomersDMI and DMO.

EXAMPLE 3

The simultaneous production of fibregrade DMT (DMT-p) andDMT-intermediate products for terephthalic acid production is shown to afurther extent in FIG. 3. For this purpose, the crystalysate flow fromthe centrifuge is distributed to mixing tanks 36 and 54. The DMTcrystalysate is mixed with methanol from the intermediate tank 52 in themixing tank 54 and supplied via the pump 55 to the centrifuge 56. Thefiltrate being produced is supplied to the methanol intermediate tank39, while the crystalysate in the mixing tank 57 is mixed with puremethanol and is supplied via the pump 58 and heat exchanger 43 to theintermediate tank 44. The DMT dissolved in methanol is further purifiedin the second recrystallisation consisting of crystallyser 45, heatexchanger 47 and pump 46. The crystal suspension is then supplied, viathe pump 48 to the washing centrifuge 49 where it is distributed into afiltrate and a crystalysate flow. The crystalysate is then melted inmelter 50 and freed from residual mother liquor while the filtrate isdirected via the condensation head of the melter 50 to the intermediatetank 52. The filtrate consisting almost exclusively of methanol is fedback via the pump 53 to the mixing tank 54 (countercurrent). The DMT-pproduced is withdrawn by means of the pump 51 to the melter 50. Itcontains less than 10 ppm of isomers and only about 2 ppm TAE and HM-BME(material flow 3.6) and represents accordingly a fibregrade DMTwith--for large scale industrial plants--hitherto unachieved purity.

EXAMPLE 4 Hydrolysis (PTA)

The hydrolysis of the DMT intermediate product according to FIG. 3(material flow 3.5) to fibre pure terephthalic acid (PTA) is explainedmore precisely with reference to FIG. 5.

The DMT intermediate product (material flow 5.1) coming from the firstrecrystallisation (FIG. 3) is mixed in mixing tank 73 with water fromthe stripping steam producer 74 in the ratio 1:1 and partiallyhydrolysed. The suspension is pumped into the hydrolysis reactor 76 bymeans of the pump 75, in which the mixture is treated with strippingsteam from the stripping steam producer 74. In this way the methanolbeing freed in the hydrolysis together with steam and the dimethyletherformed is driven off in the hydrolysis column 77 with condenser 78 andheat exchanger 79. By suitable choice of the reflux ratio, the result isattained that a methanol free water can be withdrawn at the sump of thecolumn 77 with the help of the pump 80 and can be fed back into thestripping steam producer. The condensed head product (material flow 5.2)consists chiefly of dimethylether (DME), methanol and water and can besupplied to a methanol rectification (not shown) after distillativeseparation off of the DME. The dimethylether is burned off while themethanol is fed back into the esterification (FIG. 1). From the reactor76, the terephthalic acid being produced is crystallised out in athree-step expansion crystallisation (crystallisers 81, 82 and 83) andconveyed to the centrifuge 85 by means of pump 84. The crystal slurry issupplied to the drier 86 and dried there to form pure terephthalic acid(PTA) (material flow 5.5). The mother liquor running out from thecentrifuge 85 into the collecting tank 87 is discharged, partially aswaste water (material flow 5.4) for removal of still dissolved isomers(ITA, OTA) and of foreign substances which arise for example by wear ofthe mechanically moved parts like slide ring seals etc. The largest partof the filtrate is however fed back by means of the pump 88 to thestripping vapour producer with heat exchanger 89. The water used in thehydrolysis and withdrawn in the distillate (material flow 5.2) as wellas in the waste water (material flow 5.4) is replaced by fresh deionisedwater (material flow 5.3).

The temperature in the mixing tank 73 and in the hydrolysis reactor 76amounts in the illustrated exemplifying embodiment to about 265° C. Theexpansion crystallisation was conducted at up to a temperature of 95° C.As is to be seen from Table V (material flow 5.5) the PTA has inaddition to about 100 ppm MMT still only about 40 ppm of furtherimpurities and is accordingly suitable for almost all usages.

EXAMPLE 5 Hydrolysis (PTA-p)

Should a terephthalic acid with hitherto unachieved low content of MMTand p-TA (PTA-p) be desired, the PTA in the connection to thecrystallisation can optionally be washed once again with deionisedwater. The combined production of PTA and PTA-p according to FIG. 6 isespecially advantageous. The arrangements 73 to 89 and the procedurecorrespond moreover largely to FIG. 5. Only in the crystallyser 82 is apart of the crystal slurry withdrawn by means of pump 90 and pumped intothe countercurrent washer 91, e.g. according to DE-A-1=l -=b 36 39 958,where it is washed in countercurrent with deionised water (material flow6.3). The washing takes place at a temperature of 190° C. The washedcrystal suspension is released into the crystalliser 92 and conveyedfrom there by means of the pump 93 to the centrifuge 94 for separatingoff of the mother liquor. The damp PTA is dried in the drier 95 andleaves the plant as high purity PTA-p with a total content of impuritiesinclusive of MMT and p-TA of <50 ppm (material flow 6.6, Table 6) whilethe mother liquor running off from the centrifuge 94 is combined in thetank 96 with the mother liquor running off from the centrifuge 85 and ispumped by means of pump 97 into the stripping stream producer 74.

Pump 88 conveys the mother liquor from the countercurrent washing 91back into the stripping stream producer 74.

Abbreviations

DME--Dimethylether

DMI--Dimethyl isophthalate

DMO--Dimethyl orthophthalate

DMT--Dimethyl terephthalate

DMT-p--Pure DMT (fibregrade DMT)

HM-BME--Hydroxymethylbenzoic acid methyl ester

ITA--Isophthalic acid

MM-BME--Methoxymethylbenzoic acid methyl ester

MMT--Monomethyl terephthalate

MTA--Medium pure terephthalic acid

OTA--Orthophthalic acid

p-TA--para-Toluic acid

p-TE--para-Toluic acid methyl ester (pT-ester)

p-X--para-Xylene

PTA--Pure terephthalic acid (fibregrade TA)

PTA-p--Terephthalic acid of very high purity (content of MMT and p-TA<50ppm)

TA--Terephthalic acid

TAE--Terephthalaldehydic acid methyl ester

TAS--Terephthalaldehydic acid (4-CBA)

Definitions

In the description, % and ppm amounts used relate, unless otherwiseindicated, to parts by weight

    ______________________________________                                        Legend                                                                        ______________________________________                                         1 -          Oxidiser                                                         2 -          Stripper                                                         3 -          High pressure pump                                               4 -          Heat Exchanger                                                   5 -          Esterification column                                            6 -          Intermediate tank                                                7 -          Crude ester rectification                                                     (high boiler column)                                             8 -          Evaporator                                                       9 -          Condenser                                                       10 -          Pump                                                            11 -          Crude ester rectification                                                     (low boiler column)                                             12 -          Vaporiser                                                       13 -          Condenser                                                       14 -          Pump                                                            15 -          Tank                                                            16 -          Pump                                                            17 -          Heat Exchanger                                                  18 -          Pump                                                            19 -          Reactor                                                         20 -          Distillation column                                             21 -          Vaporiser                                                       22 -          Pump                                                            23 -          Dephlegmator                                                    24 -          Reactor                                                         25 -          Heat Exchanger                                                  26 -          Circulating pump                                                27 -          Dephlegmator                                                    28 -          Mixing Tank                                                     29 -          Crystallizer                                                    30 -          Circulating Pump                                                31 -          Condenser                                                       32 -          Pump                                                            33 -          Centrifuge                                                      34 -          Tank                                                            35 -          Pump                                                            36 -          Tank                                                            37 -          Pump                                                            38 -          Centrifuge                                                      39 -          Methanol-intermediate tank                                      40 -          Pump                                                            41 -          Melter                                                          42 -          Pump                                                            43 -          Heat exchanger                                                  44 -          Intermediate tank                                               45 -          Crystalliser                                                    46 -          Pump                                                            47 -          Heat Exchanger                                                  48 -          Pump                                                            49 -          Washing centrifuge                                              50 -          Melter                                                          51 -          Pump                                                            52 -          Intermediate tank                                               53 -          Pump                                                            54 -          Mixing tank                                                     55 -          Pump                                                            56 -          Centrifuge                                                      57 -          Mixing tank                                                     58 -          Pump                                                            59 -          Evaporator                                                      60 -          Separator                                                       61 -          Tank                                                            62 -          Pump                                                            63 -          Crystalliser                                                    64 -          Condenser                                                       65 -          Pump                                                            66 -          Centrifuge                                                      67 -          Stirring Tank                                                   68 -          Pump                                                            69 -          Collecting Tank                                                 70 -          Pump                                                            71 -          Evaporator                                                      72 -          Pump                                                            73 -          Mixing Tank                                                     74 -          Stripping Vapour producer                                       75 -          Pump                                                            76 -          Hydrolysis reactor                                              77 -          Hydrolysis column                                               78 -          Condenser                                                       79 -          Heat Exchanger                                                  80 -          Pump                                                            81 -          Crystalliser                                                    82 -          Crystalliser                                                    83 -          Crystalliser                                                    84 -          Pump                                                            85 -          Centrifuge                                                      86 -          Drier                                                           87 -          Collecting Tank                                                 88 -          Pump                                                            89 -          Heat Exchanger                                                  90 -          Pump                                                            91 -          Countercurrent washer                                           92 -          Crystalliser                                                    93 -          Pump                                                            94 -          Centrifuge                                                      95 -          Drier                                                           96 -          Tank                                                            97 -          Pump                                                            ______________________________________                                    

                                      TABLE I                                     __________________________________________________________________________                Stream No.                                                                    1.1      1.2     1.3      1.4      1.5                                        Xylene to oxid.                                                                        Process air                                                                           Catalyst p-TE     Discharge gas from                         (after separation)                                                                     to oxidation                                                                          to oxidation                                                                           to oxidation                                                                           oxidation                      Components  kg/h                                                                              wt % kg/h                                                                              wt %                                                                              kg/h                                                                              wt % kg/h                                                                              wt % kg/h wt %                      __________________________________________________________________________    High boilers                                                                              --  --                    --  --   --   --                        DMT + DMI + DMO                                                                           191,8                                                                             0,58                  11244,6                                                                           22,48                                                                              118,7                                                                              0,12                      MM-BME                                379,2                                                                             0,76                                MMT                                   516,1                                                                             1,03                                HM-BME                                499,1                                                                             0,99                                TA                                    --  --                                  TAE         15,8                                                                              0,05                  1144,2                                                                            2,29 15,8 0,02                      p-TE        5189,0                                                                            15,63                 32473,3                                                                           64,92                                                                              2137,1                                                                             2,21                      TAS         --  --                    --  --   --   --                        p-TA        102,9                                                                             0,31                  --  --   102,9                                                                              0,11                      BME         2239,5                                                                            6,75                  3556,6                                                                            7,11 1266,4                                                                             1,31                      Benzoic acid                                                                              --  --                             --   --                        p-X         25318,5                                                                           76,28                          5960,1                                                                             6,17                      Acetic acid                                    441,8                                                                              0,46                      Cobalt                       32,08                                                                             6,91          --   --                        Manganese                    7,76                                                                              1,68          --   --                        Formic acid                                    220,9                                                                              0,23                      Carbon dioxide                                 1850,8                                                                             1,92                      Methanol                              136,9                                                                             0,27 474,9                                                                              0,49                      Oxygen               21720,4                                                                           22,86                 2732,3                                                                             2,83                      Nitrogen             71902,1                                                                           75,69                 71902,1                                                                            74,51                     Carbon monoxide      --  --                    462,7                                                                              0,48                      Water       133,5                                                                             0,40 1377,5                                                                            1,45                                                                              424,16                                                                            91,41                                                                              69,0                                                                              0,15 7915,2                                                                             8,20                      Light ends  --  --   --  --  --  --   --  --   900,3                                                                              0,94                      Σ Total                                                                             33191                                                                             100,00                                                                             95000                                                                             100,00                                                                            464 100,00                                                                             50019                                                                             100,00                                                                             96502                                                                              100,00                    __________________________________________________________________________                Stream No.                                                                    1.6      1.7      P.sub.1                                                     Vapour from                                                                            Steam to Oxidation product                                           stripper stripper upstream stripper                               Components  kg/h                                                                              wt % kg/h                                                                              wt % kg/h                                                                              wt % kg/h                                                                              wt % kg/h                                                                              wt %                      __________________________________________________________________________    High boilers                  2062,5                                                                            2,51                                        DMT + DMI + DMO               11101,4                                                                           13,51                                       MM-BME                        --  --                                          MMT                           18466,4                                                                           22,47                                       HM-BME                        164,3                                                                             0,20                                        TA                            8307,6                                                                            10,11                                       TAE                           394,4                                                                             0,48                                        p-TE        616,2                                                                             9,34          19722,3                                                                           24,00                                       TAS                           714,9                                                                             0,87                                        p-TA                          14330,8                                                                           17,44                                       BME         308,1                                                                             4,67          4839,9                                                                            5,89                                        Benzoic acid                                                                              --  --            82,2                                                                              0,10                                        p-X         1232,4                                                                            18,68         1232,6                                                                            1,5                                         Acetic acid 164,3                                                                             2,49          164,3                                                                             0,20                                        Cobalt      --  --            10,7                                                                              0,013                                       Manganese   --  --            2,5 0,003                                       Formic acid 82,2                                                                              1,25          82,2                                                                              0,10                                        Carbon dioxide                                                                Methanol                                                                      Oxygen                                                                        Nitrogen                                                                      Carbon monoxide                                                               Water       4193,8                                                                            63,57                                                                              3921                                                                              100,00                                                                             493,0                                                                             0,6                                         Light ends  --  --            --  --                                          Σ Total                                                                             6597                                                                              100,00                                                                             3921                                                                              100,00                                                                             82172                                                                             100,00                                      __________________________________________________________________________                Stream No.                                                                    1.8     1.9      1.10     1.11                                                Methanol to                                                                           Methanol vapour                                                                        Vapour from                                                                            Vapour from                                         oxidate heater                                                                        to esterification                                                                      esterification                                                                         crude ester flash                       Components  kg/h                                                                              wt %                                                                              kg/h                                                                              wt % kg/h wt %                                                                              kg/h wt % kg/h                                                                              wt %                      __________________________________________________________________________    High boilers                 --   --  --   --                                 DMT + DMI + DMO              73,1 0,26                                                                              1602,3                                                                             7,50                               MM-BME                                --   --                                 MMT                                   516,1                                                                              2,42                               HM-BME                                98,0 0,46                               TA                                    --   --                                 TAE                                   47,6 0,22                               p-TE                         2435,7                                                                             8,81                                                                              10853,7                                                                            50,83                              TAS                          --   --  --   --                                 p-TA                         --   --  --   --                                 BME                          665,0                                                                              2,41                                                                              1539,4                                                                             7,21                               Benzoic acid                                                                  p-X                                                                           Acetic acid                                                                   Cobalt                                                                        Manganese                                                                     Formic acid                                                                   Carbon dioxide                                                                Methanol    3613,0                                                                            100,00                                                                            32154,0                                                                           100,00                                                                             18562,9                                                                            67,15                                                                             6501,4                                                                             30,45                              Oxygen                                                                        Nitrogen                                                                      Carbon monoxide                                                               Water                        5825,6                                                                             21,07                                                                             194,5                                                                              0,91                               Light ends                   82,7 0,30                                                                              --   --                                 Σ Total                                                                             3613,0                                                                            100,00                                                                            32154,0                                                                           100,00                                                                             27645                                                                              100,00                                                                            21353                                                                              100,00                             __________________________________________________________________________                Stream No.                                                                    1.12       1.13            1.15     1.16                                      p-TE distillate                                                                          crude DMT to                                                                          1.14    Vacuum gas from                                                                        Vacuum gas from                           from crude DMT still                                                                     recrystallisation                                                                     Residue A                                                                             crude DMT still                                                                        crude ester still             Components  kg/h  wt % kg/h                                                                              wt %                                                                              kg/h                                                                              wt %                                                                              kg/h                                                                              wt % kg/h                                                                              wt %                      __________________________________________________________________________    High boilers                                                                              --    --   --  --  2261,6                                                                            36,83                                                                             --  --   --  --                        DMT + DMI + DMO                                                                           4789,8                                                                              18,16                                                                              29641,7                                                                           99,80                                                                             3167,0                                                                            51,57                                                                             47,5                                                                              2,19 265,9                                                                             13,13                     MM-BME      --    --   --  --  --  --                                         MMT         --    --   --  --  610,7                                                                             9,94                                       HM-BME      --    --   14,8                                                                              0,05                                                                              51,5                                                                              0,84                                       TA          --    --   --  --                                                 TAE         1053,2                                                                              4,00 29,7                                                                              0,10        26,5                                                                              1,22 16,9                                                                              0,83                      p-TE        18788,8                                                                             71,34                                                                              --  --          1643,6                                                                            75,59                                                                              1107,7                                                                            54,71                     TAS         --    --   --  --          --  --   --  --                        p-TA        --    --   14,8                                                                              0,05        --  --   --  --                        BME         1704,2                                                                              6,47                 352,4                                                                             16,21                                                                              262,0                                                                             12,94                     Benzoic acid                                                                  p-X                                                                           Acetic acid                                                                   Cobalt                         40,4                                                                              0,66                                       Manganese                      9,8 0,16                                       Formic acid                                                                   Carbon dioxide                                                                Methanol                               41,1                                                                              1,89 290,2                                                                             14,33                     Oxygen                                 --  --   --  --                        Nitrogen                               56,0                                                                              2,58 56,0                                                                              2,77                      Carbon monoxide                        --  --   --  --                        Water                                  6,9 0,32 26,3                                                                              1,29                      Light ends                             --  --   --  --                        Σ Total                                                                             26366,0                                                                             100,00                                                                             29701,0                                                                           100,00                                                                            6141,0                                                                            100,00                                                                            2174,0                                                                            100,00                                                                             2025,0                                                                            100,00                    __________________________________________________________________________                 Stream No.                                                                    1.17       1.18       1.19       1.20                                         Feed to    Reflux to  Distillate of                                                                            Reflux to                                    crude ester still                                                                        crude ester still                                                                        crude ester still                                                                        crude DMT still                 Components   kg/h wt %  kg/h wt %  kg/h wt %  kg/h wt %                       __________________________________________________________________________    High boilers 2261,6                                                                             3,41  --   --    --   --    --   --                         DMT + DMI + DMO                                                                            37911,9                                                                            57,21 17079,1                                                                            59,29 34479,0                                                                            59,29 7350,03                                                                            18,19                      MM-BME       --   --    --   --    --   --    --   --                         MMT          610,7                                                                              0,92  --   --    --   --    --   --                         HM-BME       66,3 0,10  7,3  0,03  14,8 0,03  --   --                         TA           --   --    --   --    --   --    --   --                         TAE          1126,3                                                                             1,70  549,5                                                                              1,91  1109,4                                                                             1,91  1616,28                                                                            4,00                       p-TE         21540,1                                                                            32,51 10121,2                                                                            35,13 20432,4                                                                            35,13 28826,35                                                                           71,34                      TAS          --   --    --   --    --   --    --   --                         p-TA         14,8 0,02  7,3  0,03  14,8 0,03  --   --                         BME          2318,6                                                                             3,50  1018,8                                                                             3,53  2056,6                                                                             3,53  2614,34                                                                            6,47                       Benzoic acid --   --                                                          p-X          --   --                                                          Acetic acid  --   --                                                          Cobalt       40,4 0,06                                                        Manganese    9,8  0,02                                                        Formic acid  --   --                                                          Carbon dioxide                                                                             --   --                                                          Methanol     331,5                                                                              0,50  20,4 0,07  9,841,4                                                                            0,07                                  Oxygen       --   --    --   --    --   --                                    Nitrogen     --   --    --   --    --   --                                    Carbon monoxide                                                                            --   --    --   --    --   --                                    Water        33,2 0,05  3,4  0,01  6,9  0,01                                  Light ends   --   --               --   --                                    Σ Total                                                                              66265,0                                                                            100,00                                                                              28807,0                                                                            100,00                                                                              58155,0                                                                            100,00                                                                              40407,0                                                                            100,00                     __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________                Stream No.                                                                    2.1      2.2       2.3       2.4        2.5                                   Residue A from                                                                         Methanol from                                                                           Residue B to                                                                            Vapour from re-                                                                          Distillate from re-                   crude ester still                                                                      filtrate evaporation                                                                    catalyst recovery                                                                       esterification                                                                           esterification                                                                column                    Components  kg/h                                                                              wt % kg/h wt % kg/h wt % kg/h  wt % kg/h  wt                  __________________________________________________________________________                                                              %                   High boilers                                                                              2261,6                                                                            36,83          876,1                                                                              91,73                                                                              --    --   --    --                  DMT + DMI + DMO                                                                           3167,0                                                                            51,57          28,7 3,01 436,6 6,04 3063,0                                                                              96,93               MM-BME      --  --                       --    --   --    --                  MMT         610,7                                                                             9,94                     --    --   --    --                  HM-BME      51,5                                                                              0,84                     2,7   0,04 44,5  1,21                TA                                       --    --   --    --                  TAE                                      --    --   --    --                  p-TE                                     1,7   0,02 1,7   0,05                TAS                                                                           p-TA                                                                          BME                                                                           Benzoic acid                                                                  p-X                                                                           Acetic acid                                                                   Cobalt      40,4                                                                              0,66           40,4 4,23                                      Manganese   9,8 0,16           9,8  1,03                                      Formic acid                                                                   Carbon dioxide                                                                Methanol             13238,0                                                                            100,00         6766,6                                                                              93,55                                                                              50,3  1,59                Oxygen                                   --    --   --    --                  Nitrogen                                 --    --   --    --                  Carbon monoxide                          --    --   --    --                  Water                                    25,4  0,35 0,5   0,02                Light ends                               --    --   --    --                  Σ Total                                                                             6141,0                                                                            100,00                                                                             13238,0                                                                            100,00                                                                             955,0                                                                              100,00                                                                             7233,0                                                                              100,00                                                                             3160,0                                                                              100,00              __________________________________________________________________________                Stream No.                                                                    2.6        2.7       2.8       2.9       2.15                                 Reflux to re-                                                                            Vapour from                                                                             Distillate from                                                                         Reflux to Residue from re-                     esterification column                                                                    reactor column                                                                          reactor column                                                                          reactor column                                                                          esterification                                                                reactor                  Components  kg/h  wt % kg/h wt % kg/h wt % kg/h wt % kg/h wt                  __________________________________________________________________________                                                              %                   High boilers                                                                              --    --   --   --   --   --   --   --   1966,9                                                                             78,58               DMT + DMI + DMO                                                                           2322,5                                                                              96,93                                                                              204,3                                                                              3,15 834,6                                                                              53,95                                                                              1733,4                                                                             53,95                                                                              474,6                                                                              18,96               MM-BME      --    --   34,0 0,52 345,1                                                                              22,31                                                                              716,8                                                                              22,31                                                                              --   --                  MMT         --    --   --   --   --   --   --   --   --   --                  HM-BME      33,7  1,41 38,6 0,60 313,6                                                                              20,27                                                                              651,3                                                                              20,27                                                                              7,4  0,30                TA          --    --   --   --   --   --   --   --                            TAE         --    --   --   --   --   --   --   --                            p-TE        1,3   0,05 50,1 0,77 26,2 1,69 54,3 1,69                          TAS                                                                           p-TA                                                                          BME                                                                           Benzoic acid                                                                  p-X                                                                           Acetic acid                                                                   Cobalt                                               40,4 1,61                Manganese                                            9,8  0,39                Formic acid                                          --   --                  Carbon dioxide                                       --   --                  Methanol    38,1  1,59 6142,0                                                                             94,73                                                                              27,5 1,78 57,2 1,78 3,9  0,16                Oxygen      --    --   --   --                                                Nitrogen    --    --   --   --                                                Carbon monoxide                                                                           --    --   --   --                                                Water       0,4   0,02 15,0 0,23                                              Light ends  --    --   --   --                                                Σ Total                                                                             2396,0                                                                              100,00                                                                             6484,0                                                                             100,00                                                                             1547,0                                                                             100,00                                                                             3213,0                                                                             100,00                                                                             2503,0                                                                             100,00              __________________________________________________________________________                                 Stream No.                                                                    2.16                                                                          Sump product of re-                                                           esterification column                                                                            2.10                                                                             2.11                                                                             2.12                                                                             2.13                                                                             2.14                               Components  kg/h  wt % kg/h                                                                              wt %                                                                              kg/h                                                                             kg/h                                                                             kg/h                                                                             kg/h                                                                             kg/h              __________________________________________________________________________                     High boilers                                                                              142,5 95,00        1530                                                                             4580                                                                             795                                                                              4743                                                                             1590                               DMT + DMI + DMO                                                                           7,5   5,00         Methanol vapour                                MM-BME                         see stream no. 2.2                             MMT                                                                           HM-BME                                                                        TA                                                                            TAE                                                                           p-TE                                                                          TAS                                                                           p-TA                                                                          BME                                                                           Benzoic acid                                                                  p-X                                                                           Acetic acid                                                                   Cobalt                                                                        Manganese                                                                     Formic acid                                                                   Carbon dioxide                                                                Methanol                                                                      Oxygen                                                                        Nitrogen                                                                      Carbon monoxide                                                               Water                                                                         Light ends                                                                    Σ Total                                                                             150,0 100,00                                     __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________           Stream No.                                                                    3.1       3.2      3.3       3.4                                              crude DMT for 1st                                                                       Pure methanol for                                                                      Crystal slurry from                                                                     Filtrate A to                                    recrystallisation                                                                       recrystallisation                                                                      isomers discharging                                                                     isomers discharging                       Components                                                                           kg/h wt % kg/h                                                                              wt % kg/h wt % kg/h wt % kg/h                                                                              wt %                        __________________________________________________________________________    DMT    29463,4                                                                            99,2          1172,9                                                                             35,16                                                                              1219,4                                                                             2,05                                 DMI    104,0                                                                              0,35          21,95                                                                              0,66 125,07                                                                             0,21                                 DMO    74,3 0,25          15,55                                                                              0,47 89,22                                                                              0,15                                 HM-BME 14,8 0,05          3,15 0,09 17,78                                                                              0,03                                 TAE    29,7 0,10          6,30 0,19 35,75                                                                              0,06                                 p-TA   14,8 0,05          3,15 0,09 17,78                                                                              0,03                                 Methanol                                                                             --   --   55873                                                                             100,00                                                                             2113,0                                                                             63,34                                                                              57986,0                                                                            97,47                                Σ Total                                                                        29701,0                                                                            100,00                                                                             55873                                                                             100,00                                                                             3336,0                                                                             100,00                                                                             59491,0                                                                            100,00                               __________________________________________________________________________           Stream No.                                                                    3.5        3.6      3.7                                                       Intermediate product                                                                     DMT-p from                                                                             Intermediate product                                      for hydrolysis                                                                           melter   for hydrolysis                                     Components                                                                           kg/h  wt % kg/h                                                                              wt % kg/h  wt % kg/h                                                                             wt % kg/h                                                                             wt %                         __________________________________________________________________________    DMT    21917,0                                                                             99,991                                                                             7499,9                                                                            99,999                                                                             21902,14                                                                            99,92                                        DMI    0,85  39 ppm                                                                             0,03                                                                              4,3 ppm                                                                            7,38  337 ppm                                      DMO    0,61  28 ppm                                                                             0,02                                                                              3,1 ppm                                                                            5,27  240 ppm                                      HM-BME 0,15  6 ppm                                                                              traces                                                                            0,7 ppm                                                                            1,05  48 ppm                                       TAE    0,24  11 ppm                                                                             traces                                                                            1,2 ppm                                                                            2,11  96 ppm                                       p-TA   0,15  6 ppm                                                                              traces                                                                            0,7 ppm                                                                            1,05  48 ppm                                       Methanol                                                                             --    --   --  --   --    --                                                                      Only for MTA                                                                  production                                         Σ Total                                                                        21919,0                                                                             100,00                                                                             7500,0                                                                            100,00                                                  __________________________________________________________________________

                                      TABLE IV                                    __________________________________________________________________________           Stream No.                                                                    4.1       4.2        4.3       4.4     4.5                                    Filtrate A from 1st                                                                     Methanol vapour from                                                                     Crystal slurry to 1st                                                                   Isomers to                                                                            Methanol vapour from                   recrystallisation                                                                       filtrate A evaporation                                                                   recrystallisation                                                                       burning isomer evaporation              Components                                                                           kg/h wt % kg/h  wt % kg/h wt % kg/h                                                                              wt %                                                                              kg/h  wt %                      __________________________________________________________________________    DMT    1219,4                                                                             2,05            1172,9                                                                             35,16                                                                              46,5                                                                              16,49                               DMI    125,07                                                                             0,21            21,95                                                                              0,66 103,12                                                                            36,57                               DMO    89,22                                                                              0,15            15,55                                                                              0,47 73,67                                                                             26,12                               HM-BME 17,78                                                                              0,03            3,15 0,09 14,63                                                                             5,19                                TAE    35,75                                                                              0,06            6,30 0,19 29,45                                                                             10,44                               p-TA   17,78                                                                              0,03            3,15 0,09 14,63                                                                             5,19                                Methanol                                                                             57986,0                                                                            97,47                                                                              54387,0                                                                             100,00                                                                             2113,0                                                                             63,34                                                                              --  --  1486,0                                                                              100,00                    Σ Total                                                                        59491,0                                                                            100,00                                                                             54387,0                                                                             100,00                                                                             3336,0                                                                             100,00                                                                             282,0                                                                             100,00                                                                            1486,0                                                                              100,00                    __________________________________________________________________________

                                      TABLE V                                     __________________________________________________________________________           Stream No.                                                                    5.1        5.2                                                                DMT intermediate                                                                         Distillate from hydro-                                                                   5.3      5.4      5.5                                   product to hydrolysis                                                                    methanol distillation                                                                    Deionised water                                                                        Waste water                                                                            PTA                            Components                                                                           kg/h  wt % kg/h  wt % kg/h                                                                              wt % kg/h                                                                              wt % kg/h wt %                      __________________________________________________________________________    DMT    21917,0                                                                             99,991                                                           DMI    0,85  39 ppm                                                           DMO    0,61  28 ppm                                                           HM-BME 0,15  6 ppm                                                            TAE    0,24  11 ppm                                                           p-TA   0,15  6 ppm                    0,1 15 ppm                                                                             0,05 <3 ppm                    Methanol          6927,0                                                                              51,84         --  --   --   --                        ITA                                   0,53                                                                              78 ppm                                                                             0,21 11 ppm                    OTA                                   0,37                                                                              54 ppm                                                                             0,12 6 ppm                     TA                                    1,8 264 ppm                                                                            18747,44                                                                           99,98                     TAS                                   --  --   0,3  16 ppm                    Water             5995,0                                                                              44,87                                                                              17000,0                                                                           100,00                                                                             6799,30                                                                           99,87                                                                              --   --                        DME               439,0 3,29 --  --   --  --   --   --                        MMT               --    --   --  --   5,9 867 ppm                                                                            1,88 100 ppm                   Σ Total                                                                        21919,0                                                                             100,00                                                                             13361,0                                                                             100,00                                                                             17000,0                                                                           100,00                                                                             6808,0                                                                            100,00                                                                             18750,0                                                                            100,00                    __________________________________________________________________________

                                      TABLE VI                                    __________________________________________________________________________           Stream No.                                                                    6.1        6.2                                                                DMT intermediate                                                                         Distillate from hydro-                                                                   6.3       6.4         6.5                               product to hydrolysis                                                                    methanol distillation                                                                    Deionised water                                                                         Waste water PTA                        Components                                                                           kg/h  wt % kg/h  wt % kg/h wt % kg/h  wt %  kg/h  wt                   __________________________________________________________________________                                                             %                    DMT    21917,0                                                                             99,991                                                           DMI    0,85  39 ppm                                                           DMO    0,61  28 ppm                                                           HM-BME 0,15  6 ppm                                                            TAE    0,24  11 ppm                                                           p-TA   0,15  6 ppm                     0,128 18 ppm                                                                              0,03  2 ppm                Methanol          6927,0                                                                              51,84          --    --    --    --                   ITA                                    0,59  87 ppm                                                                              0,14  10 ppm               OTA                                    0,38  46 ppm                                                                              0,07  5 ppm                TA                                     1,8   264 ppm                                                                             14048,14                                                                            99,98                TAS                                    --    --    0,22  16 ppm               Water             5995,0                                                                              44,87                                                                              17000,0                                                                            100,00                                                                             6799,21                                                                             99,87 --    --                   DME               439,0 3,29 --   --   --    --    --    --                   MMT               --    --   --   --   5,9   867 ppm                                                                             1,4   100 ppm              Σ Total                                                                        21919,0                                                                             100,00                                                                             13361,0                                                                             100,00                                                                             17000,0                                                                            100,00                                                                             6808,0                                                                              100,00                                                                              14050,0                                                                             100,00               __________________________________________________________________________                              Stream No.                                                                    6.6                                                                           PTA-p                                                                  Components                                                                           kg/h                                                                              wt % kg/h                                                                             wt %                                                                              kg/h                                                                             wt %                                                                              kg/h                                                                             wt %                                                                              kg/h                                                                             wt                 __________________________________________________________________________                                                               %                                     DMT                                                                           DMI                                                                           DMO                                                                           HM-BME                                                                        TAE                                                                           p-TA   <0,01                                                                             0,4 ppm                                                            Methanol                                                                             --  --                                                                 ITA    <0,01                                                                             2,4 ppm                                                            OTA    <0,01                                                                             1,4 ppm                                                            TA     4699,8                                                                            99,996                                                             TAS    0,07                                                                              16 ppm                                                             Water  --  --                                                                 DME    --  --                                                                 MMT    0,11                                                                              25 ppm                                                             Σ Total                                                                        4700,0                                                                            100,00                                          __________________________________________________________________________

We claim:
 1. A process for the production of a DMT intermediate productintended for working up to medium pure or pure dimethyl terephthalate(DMT) and/or medium pure or pure terephthalic acid (TA), theintermediate product having a total content of impurities of less than1% by weight and a terephthalaldehydic acid methyl ester (TAE) contentof less than 100 ppm, comprising:(a) jointly oxidizing a mixturecontaining predominantly para-xylene (p-X) and paratoluic acid methylester (p-TE) in the liquid phase with an oxygen-containing gas; (b)esterifying the acids produced in the oxidative step (a) with methanol;(c) distilling the reaction mixture produced in the esterification step(b) into:(I) a p-TE rich fraction, (II) a fraction containing more than99% by weight DMT and its isomers, and (III) a high boiling residuefraction; and (d) recycling the p-TE fraction I to oxidation step (a),wherein the residue fraction III has a DMT content of 15 to 70% byweight and the DMT fraction II is purified by single solventrecrystallization to the DMT intermediate product such that the amountsof hydroxymethyl-benzoic acid methyl ester (HM-BME) andterephthalaldehydic acid methyl ester (TAE) amount together to less than200 ppm in the DMT intermediate product.
 2. A process according to claim1, wherein the combined amounts of HM-BME and TAE in the DMT fraction IIare less than 0.2% by weight, and the amounts of HM-BME and TAE in theDMT intermediate product are less than 50 ppm.
 3. A process according toclaim 1 wherein 3 to 7% by weight DMT, relative to the sum of allcomponents supplied to oxidation step (a) in the liquid phase, issupplied additionally to oxidation step (a) together with thepara-xylene and the p-TE fraction I.
 4. A process according to claim 3,wherein the amount of DMT supplied additionally to the oxidation isobtained by working up residue fraction III.
 5. A process according toclaim 1, wherein, for the distillation of the esterification products,the ester mixture is separated in a first rectification column into asump product forming the high boiling residue fraction III and a headproduct, with the pressure in the rectification column amounting to 0.15to 0.5 bar at the sump and 0.1 to 0.4 bar at the head and the refluxratio amounting to between 0.2 and 1, and the head product is split inthe second rectification column into the DMT fraction II as sump productand the p-TE fraction I as head product, with the pressure in the secondrectification column amounting to 0.2 to 0.7 bar at the sump and to 0.1to 0.5 bar at the head and the reflux ratio amounting to between 0.5 and2.5.
 6. A process according to claim 2, wherein the combined amounts ofH-BME and TAE in the DMT fraction II are less than 0.1% by weight.
 7. Aprocess according to claim 6, wherein 3 to 7% by weight DMT, relative tothe sum of all components supplied to oxidation step (a) in the liquidphase, is supplied additionally to oxidation step (a) together with thepara-xylene and the p-TE fraction I.
 8. A process according to claim 7,wherein the amount of DMT supplied additionally to the oxidation isobtained from residue fraction III.
 9. A process according to claim 8,wherein for the distillation of the esterification products, the estermixture is separated in a first rectification column into a sump productforming in the high boiling residue fraction III and a head product,with the pressure in the rectification column amounting to 0.15 to 0.5bar at the sump and 0.1 to 0.4 bar at the head and the reflux ratioamounting to between 0.2 and 1, and the head product is split in asecond rectification column into the DMT fraction II as sump product andthe p-TE fraction I as head product, with the pressure in the secondrectification column amounting to 0.2 to 0.7 bar at the sump and to 0.1to 0.5 bar at the head and the reflux ratio amounting to between 0.5 and2.5.